1. Field of the Invention
The present invention relates to an optical film in which a polarizing plate and a retardation film are laminated. Specifically, the present invention relates to a display system, such as a liquid crystal display, a PDP, and a CRT, using the above-mentioned optical film. Especially an optical film of the present invention is preferably used for a liquid crystal display operating in IPS mode.
2. Description of the Background Art
Conventionally, as a liquid crystal display, there has been used a liquid crystal display in so-called TN mode in which a liquid crystal having a positive dielectric anisotropy is twisted and homogeneous aligned between substrates mutually facing to each other. However, in TN mode, even if black display is performed, optical leakage resulting from birefringence caused by liquid crystal molecule near a substrate made it difficult to obtain perfect display of black color owing to driving characteristics thereof. On the other hand, in a liquid crystal display in IPS mode, since liquid crystal molecule has almost parallel and homogeneous alignment to a substrate surface in non-driven state, light passes through the liquid crystal layer, without giving almost any change to a polarization plane, and as a result, arrangement of polarizing plates on upper and lower sides of the substrate enables almost perfect black display in non-driven state.
However, although almost perfect black display may be realized in normal direction to a panel in IPS mode, when a panel is observed in a direction shifted from normal direction, inevitable optical leakage occurs caused by characteristics of a polarizing plate in a direction shifted from an optical axis of the polarizing plates placed on upper and lower sides of the liquid crystal cell, as a result, leading to a problem of narrowing of a viewing angle.
In order to solve this problem, a polarizing plate is used in which the shift of a geometric axis of a polarizing plate given when observed from oblique direction is compensated by a retardation film. Polarizing plates providing such effect are disclosed in Japanese Patent Laid-Open Publication No. H4-305602, and Japanese Patent Laid-Open Publication No. H4-371903 official gazette. However, retardation films conventionally known have not been able to easily realize sufficient wide viewing angles.
In a polarizing plate described in the above-mentioned Japanese Patent Laid-Open Publication No.H4-305602, a retardation film is used as a protective film for a polarizer. However, although in the polarizing plate concerned viewing angle characteristics satisfactory in usual operating environment are obtained, protective film to which a polarizer is laminated is also deforms by dimensional variation of the polarizer under conditions of high temperature and high humidity. Thus, a retardation value of a retardation film used as a protective film would be shifted from a desired value, and a problem occurred that a stable effect could not be maintained.
On the other hand, in Japanese Patent Laid-Open Publication No. H4-371903,a retardation film is laminated to a polarizing plate in which a triacetyl cellulose film (TAC film) generally used as a protective film is applied. In this case, since a direct stress does not affect to a retardation film, a stable retardation value of the retardation film is obtained. However, a non-negligible retardation value of a TAC film makes design of a retardation film difficult that can compensate an axial shift. Moreover, a dimensional variation of a polarizer caused under conditions of high temperature or high humidity varies a retardation value of a TAC film as mentioned above, thus a desired purpose cannot be attained.